The present invention relates to an apparatus for reducing, enlarging and projecting image information suitable for securing a constant luminous intensity.
As an example of an apparatus for projecting light provided with image information in a reduced or enlarged form, there is a laser marker. The laser marker is used to mark a product name, control code, etc. on an object such as an IC by a laser beam.
In a masking type laser marker a laser beam is radiated onto the surface of an IC or the like through the mask on which marking information is formed for printing the marking information thereon. In changing the marking information of the mask type markers, it is necessary to exchange a metal mask having the required information bored therein, or a glass plate having information formed as an etching image thereon. Therefore, this type of masking marker is not economical, nor is convenient in respect to the mask control, manufacture, interchange, manufacturing cost, labor saving, etc.
On the other hand, there has also been developed a typewriter type marker, in which the characters, numerals, and patterns prepared in advance can be selected by operating an optical system to print them in desired positions. Nevertheless, this type of marker still has a drawback that the marker uses only the prepared patterns and characters, the optical system required for its operation are complicated, making its high-speed performance difficult.
As shown in Japanese Laid-open Patent Publication Hei 1-11088(1989) corresponding to U.S. Pat. No. 4,818,835, a liquid crystal type laser marker has been developed wherein marking information is formed on a liquid crystal mask through which laser beams is caused to pass to form an image on an object. The marking information can be displayed electronically as an liquid crystal image. Thus it is possible to save labors to manufacture, replace, and control masks as required in the case of the conventional masking markers, and there is an advantage that the expenditure for masks can significantly be reduced. As another example of image reduction and enlargement projectors related to the present invention, there is an image projector, such as a liquid crystal projection TV. This system is such that an image is formed on a liquid crystal element as in the case of the laser marker, and that by transmitting light through this liquid crystal element, image information is provided to the light for projecting the image. A white light source is used, and the light provided with the image information is projected onto a screen in an enlarged form by a projection lens. In a color image projector, a light from the light source is separated into the three prime colors, R, G, and B, and liquid crystal elements are respectively arranged in each of the light paths for these three prime colors. Then, the image information is given to each of the three color lights, and the lights are resynthesized.
As compared with the mask type markers, the conventional liquid crystal type marker has an advantage that the masking information can easily be replaced. This type of markers, however, still has drawbacks given below. In a liquid crystal type marker, an output beam from a pulse laser using an Nd:YAG rod is ordinarily employed for marking after having converted this output beam into a linearly polarized light by the use of a Brewster's window. If the frequency of the pulse oscillation is raised to speed up the marking, a temperature gradient is generated in the YAG rod due to the pumping energy of a flash lamp required therefor. Hence, an intensive tensile stress is exerted on the regions in the vicinity of the periphery of the cylindrical rod.
The increased pumping power causes the YAG rod to be deformed, resulting in a thermal lensing effect by which in turn the YAG rod causes the resonance system to be out of focus. However, the thermal lensing effect does not give any hindrance to the pulse pumping system such as a laser marker. The strain generated by the inner stress brings about a birefringence phenomenon having different refractive indices in the radial direction nr and in the peripheral direction n.phi. at an arbitrary point P. Since an image formed by a liquid crystal is a resultant image display which can be obtained by rotating the plane of polarization of the linearly polarized light output by the electrical field applied to the liquid crystal, it is required that the incident light is always a linearly polarized light which is maintained at a constant level. However, if a birefringence phenomenon is generated, elliptically polarized light is generated in the portion. As a result, the intensity distribution in the plane of the linear polarization is fluctuated, and an intensity distribution of the output, which is so-called a cross mode, appears. If such a phenomenon occurs, the laser beam intentionally generated is used only the portion showing the cross shape, and the marking is performed only partially.
For the marking on an IC, etc., the amount of information has increasingly become greater year after year due to the diversification of product name, such as ASIC (Application Specified IC), lot control, production control, etc. Therefore, the reduction of applicable luminous energy by a 40 to 50% is not desirable because the marking area becomes smaller than that of the conventional mask type marking. To increase the laser output is not economical because the power source and the YAG rod size must be made larger.
For the image projector, i.e., the liquid crystal projection TV, there has been proposed a system such that the light from the light source is separated by a beam splitter into two polarized light components, P wave and S wave, to obtain linearly polarized light, and only either one of them is used for the marking. In this case, the luminous energy is reduced almost by half, and a problem is encountered that a bright image cannot be obtained.